Format the Markdown with prettier in make fmt and make fmt-check (closes #215)

`make fmt` formatted only Go, so the org's Markdown settings were unenforced and Markdown was wrapped by hand. prettier, pinned in `package.json` and `yarn.lock` beside ESLint, now formats the Markdown with `.prettierrc` (4-space tabs, `proseWrap: always`). It runs only in Docker: `make fmt` writes the formatted files back without a bind mount, and `make fmt-check`, `make check` and the image build fail on unformatted Markdown. The image build's lint stage now runs the Go format check directly and no longer installs `make`. `README.md` and `TODO.md` are reformatted with no word changed: the README reflows from 72 to 80 columns.

Model: opus-5-5

.gitignore

@@ -15,7 +15,7 @@ bin/
 # Go vendor directory
 vendor/
 
-# ESLint and its dependencies, installed from yarn.lock
+# ESLint, prettier and their dependencies, installed from yarn.lock
 node_modules/
 
 # IDE specific files